1. Field of the Invention
This invention relates generally to floating, vibrating screeds for striking off and consolidating freshly poured concrete. More particularly, our invention relates to a portable, dual-operator screed that uses laser systems for automatic grade control. Pertinent prior art relating to screeds is found in U.S. Class 404, subclasses 114-118, and relevant prior art automatic grade control devices are found in Class 404, Subclasses 84.1, 84.5.
2. The Prior Art
As plastic concrete is placed during construction, it must be appropriately finished to give it a smooth, correctly textured surface. Numerous finishing devices, including screeds, have long been in use throughout the industry for treating plastic concrete. Known prior art systems include "bull" floats, finishing boards, strikeoffs, pains, plows, blades, and the like. A bull float typically comprises a flat, wooden board attached to a handle, much like a broom handle. These floats are manipulated by a single worker. Strikeoffs initially contact the jagged and irregular surfaces of unfinished plastic concrete with a rigid edge to initially form, level, and grade. In addition, modern screeds use powered vibrators to vigorously vibrate the screed. Vibration helps settle the concrete and eliminate entrapped air voids. Further, vibrational screeding densifies and compacts freshly poured concrete, drawing out excess water and increasing the resultant structural integrity.
Large cable-pulled, vibrating screeds extend between and rest upon the forms between which the plastic concrete is actually confined. Such screeds often employ automatic grade controlling apparatus responsive to an external laser source for leveling the surface. Smaller floating screeds do not ride on forms, since they are lightweight and they not sink deeply into the wet concrete. Such lightweight, portable screeds can be controlled by a single worker, and they are relatively easy to transport and deploy at the job. Even smaller floating screeds are equipped with powered vibrators.
A prior art, self-propelled "triangular truss" screed that rides upon forms, is seen in U.S. Pat. No. 4,349,328. U.S. Pat. No. 4,798,494 discloses a floating vibratory screed that finishes concrete with or without forms. U.S. Pat. Nos. 4,316,715, 4,363,618, and 4,375,351 are also relevant to the general technology discussed herein. All the above patents have been assigned to the same assignee as the present case.
U.S. Pat. No. 5,288,166, issued Feb. 22, 1994 and owned by the same assignee as in this case, discloses an automatic grading control screed having a transverse finishing mechanism transported by skis. Vertical supporting towers support the skis at periodic intervals. Each tower comprises a pair of stanchions disposed on opposite sides of the finishing tool. Suitable control cylinders raise and lower the stanchions to elevate or lower the tool and control finishing level. Tile cylinders are controlled by a beacon laser.
U.S. Pat. No. 5,328,295 owned by Allen Engineering Corporation discloses a finishing tool controlled by towers comprising a pair of extensible, spaced-apart stanchions hinged to lower skis. Suitable cylinders associated with each tower control elevation. Laser sensors detect a preestablished laser beacon for automatic level control.
U.S. Pat. Nos. 4,650,366 and 4,386,901 disclose screeds capable of formless, self-supporting or floating operation. The latter reference discloses a relatively heavy triangular truss screed adapted to be operated by two workers without the use of forms. U.S. Pat. No. 4,650,366 discloses a lightweight, portable-vibrating screed including a central, extruded beam element.
Another prior art floating screed of general relevance is disclosed in a video tape produced by the American Concrete Institute and The Portland Cement Association, entitled "Finishing Concrete Flatwork," that bears a Copyright date of 1984. Other prior art screeds, generally of the "form-riding" type, include those screeds disclosed in U.S. Pat. Nos. 4,340,351; 4,105,355; 2,651,980; 2,542,979; 3,095,789; and 4,030,873.
Lasers are commonplace on the modern construction site. They are employed in surveying, earthwork, and general layout operations. U.S. Pat. No. 4,861,189 and 4,854,769 disclose a system for paving inclined and/or curved surfaces. This system employs anchor vehicles and paving vehicles. The paving vehicles are secured to the anchor vehicles by wires. The connections of the wires to the anchor vehicles are controlled by a laser-sensing device. Microcomputers control the shape of the paving devices to create compound and complex curves in paved surfaces.
Devices employing a vehicle with a boom terminating in a screed are disclosed in U.S. Pat. Nos. 5,039,249 and 4,930,935. Each of these patents relates to an anchor vehicle and a telescoping boom extending horizontally from the vehicle. The boom terminates in a screeding device that may also employ augers and vibrators.
U.S. Pat. No. 4,978,246 discloses an apparatus and method for controlling laser-guided machines.
U.S. Pat. No. 4,752,156 discloses a manually operated laser guided portable screed having a pair of laser sensors. Screed height is established in response to a signal from the laser sensor. The above mentioned devices use a stationary laser beacon that projects laser light in a 360 degree plane.
Known prior art screeding devices tend to be either relatively large, heavyweight machines that are cable driven over forms, or relatively smaller one-person devices that, while lightweight and portable, lack many features found previously on larger units. There exists a need for a "middle-of-the road" screed that is lightweight and portable, but which accommodates automatic grade control laser leveling. A suitable portable automatic grade control screed may be handled by up to two workers without cables or winches.